1. Field of the Invention
This invention relates to instruments and methods for the non-invasive quantitative measurement of constituents in material samples, such as protein in wheat and glucose levels in a test subject's blood. Specifically, this invention relates to a novel near-infrared quantitative measurement instrument which utilizes harmonic wavelengths of light emitting diodes (LEDs).
2. Description of the Background Art
The use of LEDs and IREDs as energy sources for near-infrared measurements is a well established art. For example, thousands of TREBOR-90/XL Wheat and Barley Testers, which use IREDs as energy sources, are currently being used in country elevators for measuring the protein and moisture content in wheat and barley. Similarly, over 10,000 FUTREX-5000 Body Composition Analyzer Instruments, which also utilize IREDs, are currently being used in medical institutions, health clubs and sporting teams for measuring percent body fat. Also, a combination of LEDs and IREDs are currently being used in non-invasive near-infrared quantitative analysis instruments to assess the chemical composition of the blood, such as measurement of blood glucose levels.
One common limitation of the current generation of instruments which use LEDs/IREDs is that they are generally limited to wavelengths below approximately 1100 nanometers. This limitation is primarily due to the fact that the longest wavelengths emitted by commercially available, low cost LEDs/IREDs is typically approximately 950 nanometers. Even with the use of narrow bandpass filters located outside the IRED's half power bandwidth (see U.S. Pat. No. 4,286,327, incorporated herein by reference), typical IREDs do not provide a practical means of making measurements above approximately 1050 nanometers.
Although there are commercially available IREDs having wavelengths between 1000 and 1700 nanometers, such energy sources are extremely expensive and have a very low power output. For example, Model IR-1300 (UDT Sensors, Inc.) is an IRED that emits 20 microwatts of optical energy at 1300 nm and costs more than $30. In comparison, a typical LED emits approximately 1000 times more energy and costs less than $0.30.
As illustrated in FIG. 1, the spectrum ranges in the vicinity of 1200 to 1800 nanometers can be very important in performing quantitative measurements. This results from the fact that some absorption peaks for fat, starch and protein do not overlap the dominant water absorptions in this region. For example, at 1200 nm there is a relatively strong fat absorption band with almost no interfering absorption occurring from water. In most other regions of the spectrum, the infrared energy absorption by water significantly overlaps with the infrared energy absorption by other organic constituents such as oil, starch and protein. Thus, accurately performing quantitative measurements of such organic constituents in these lower wavelengths can be hindered by interference with water absorption.
FIG. 2 illustrates the fact that, in lower wavelength regions, infrared energy absorption by water significantly overlaps with the absorption by other organic constituents. In contrast, water absorption at 1200 and between 1600 and 1800 nanometers do not significantly interfere which makes the measurements of protein, oil/fat, starch and other constituents considerably attractive in these regions.
For example, U.S. Pat. No. 5,028,787, incorporated herein by reference, teaches a method of performing near-infrared noninvasive measurement of blood glucose levels using energy in the 600 to 1100 nanometer spectrum region. Constituent absorptions in this region are weaker than in the 1200 nanometer region. As a consequence, measurement of organic constituents in products containing water often result in major mutual interference from water. Thus, although glucose measurements in the 600 to 1100 nanometer region of the spectrum are practical, such measurements are necessarily more complex than potential measurement at 1200 or 1600-1800 nanometers.
Thus, there is a great need for a near-infrared quantitative measurement instrument having a solid state energy source (IRED) which can provide reasonable energy in the 1200 to 1800 nanometer region and yet which is reasonably priced.